1. Field of the Invention
The present invention relates to printing methods and apparatus and in particular to methods of and apparatus for printing a line of characters on a record by the differentially timed operation of a line of print hammers as the characters of a font pass predetermined columnar positions of the record.
2. Description of the Prior Art
Printing apparatus is well known in which, for example, a line of type-wheels is rotated while supported adjacent a record medium such as a paper web, a transfer medium, such as a inked ribbon being interposed between the web and the type-wheels. Behind the record medium a line of print hammers, one for each wheel, is arranged parallel to the line of the wheels. Thus, in order to print a line of characters, each print hammer is actuated as the required character on its associated print wheel arrives at the print line position on the record medium. Actuation of a hammer causes the record medium to be brought against the transfer medium which in turn is brought against the corresponding print wheel, thereby causing an impression of the character at the printing position to be recorded on the web. It will be seen that the number of characters or columnar positions in a line is set by the number of type-wheels and also that a separate hammer is required for each wheel. Hence, a printing apparatus of this kind is limited to the printing of, effectively, one size of type only and, in consequence, to a prescribed columnar pitch for the print lines.
It has also been proposed to use, instead of a line of type-wheels, a belt or chain carrying the type font, the chain moving along a track which is parallel to the line of hammers. Because the character movement is along the line to be printed, it will be seen that a very limited variation in columnar position of printer characters appears to be possible. However, in practice the variation permissible is insufficient to permit an appreciable variation in the number of characters which may be recorded in a line of given length.
As the cost of material for record purposes has risen, it has become more important to be able to reduce the costs of printing multiple copies of records. It has become well worth-while, for example, to produce purely record copies of documents on a reduced scale as compared with working copies, and it will be realised that a printing apparatus which can provide a printed output in one or other of, say, two different fonts is highly desirable.